Indonesia sank the empty fishing vessels at several sites Wednesday, local media reported, including a large Chinese ship detained in 2009 for fishing in Indonesia’s exclusive economic zone in the South China Sea.

Boats from Vietnam, Thailand and the Philippines which had been caught fishing illegally in Indonesian waters were also sunk, with some blown up using dynamite.

Indonesian President Joko Widodo launched the campaign to clamp down on illegal fishing soon after taking office in October and several foreign trawlers had already been sunk, but it was the first time a Chinese boat was targeted.

Jakarta had been accused in the past of turning a blind eye to illegal Chinese fishing activities in Indonesian waters, not wanting to anger its vastly more powerful trading partner.

However Beijing was unhappy at the move.

“China is gravely concerned about relevant reports, and is asking the Indonesian side to make clarifications,” foreign ministry spokesman Hong Lei told reporters.

“We hope that the Indonesian side can press ahead with fishery cooperation in a constructive manner and safeguard the legitimate rights and interests of Chinese companies.”

Indonesia does not have overlapping territorial claims with Beijing in the hotly contested South China Sea, home to vital shipping lanes and believed to be rich in oil and gas, unlike several other Asian nations.

But Jakarta has objected to China’s nine-dash line — the demarcation Beijing uses on maps to demonstrate its claim to almost the whole of the sea — as it overlaps with Indonesia’s exclusive economic zone around Natuna, a string of islands rich in fishing stocks on the far northwest fringe of the archipelago.

Indonesia has defended its policy of seizing and destroying illegal fishing boats and Fisheries Minister Susi Pudjiastuti told The Jakarta Post newspaper that sinking vessels was for “the welfare of our fishermen”.

Widodo has said illegal fishing costs Southeast Asia’s biggest economy billions of dollars in lost revenues every year. He hopes that increased earnings from fishing can boost economic growth, which has sunk to a five-year low.

Authorities also say foreigners illegally fishing in Indonesia are partly responsible for massive damage to the environment due to the widespread use of explosives and cyanide.